OSNR margin monitoring for optical coherent signals

ABSTRACT

A method and a device are provided for monitoring OSNR system margin in optical networks which relies on the relationship that exists between the OSNR value and the ESNR value.

TECHNICAL FIELD

The present invention relates to communication systems using coherentsignals and in particularly to monitoring Optical Signal to Noise Ratio(OSNR) margins.

BACKGROUND

Deployment of high speed transparent and reconfigurable optical networksrequires effective, flexible and robust Optical Performance Monitoring(OPM) techniques in order to ensure high quality of service as well ashigh level of resiliency.

The adoption of optical coherent detection, in which the carrier phaseand amplitude are recovered at the receiver-side and down-converted tothe electrical domain (as opposed to direct detection, in which thephase information is lost), provides an additional degree of freedom toencode and transmit information and therefore a gain in spectralefficiency. Most importantly, this lossless optical-to-electrical signalconversion offers dramatic boost to the applicability of Digital SignalProcessing (DSP), following high speed analog to digital conversion.

At the transmitter-side, the DSP may be used mainly as follows:

-   -   1. To implement advanced Dual Polarization (DP) modulation        formats (for example DP-BPSK, DP-QPSK, DP-8QAM, DP-16QAM) in        order to carry more bits per symbol;    -   2. To enhance spectral efficiency of multi-channel transmission        systems by employing techniques such as Nyquist pulse shaping or        Orthogonal Frequency Division Multiplexing (OFDM);    -   3. To implement pre-distortion techniques in order to enhance        the resilience of signal propagation to fiber impairments; and    -   4. To apply software-defined modulation in order to adapt the        signal to time/spatial-varying properties of the communication        channel and to varying transmission capacity requirements.

At the receiver-side, the DSP may be used mainly to:

-   -   1. Ease the requirements of the optical receiver, making        coherent reception more cost effective, as complexity may be        shifted from the optical domain to the electrical domain (via        digital compensation of the frequency carrier offset and the        optical phase noise);    -   2. Compensate distortions caused by signal propagation via the        optical network (Chromatic Dispersion (“CD”), Polarization Mode        Dispersion (“PMD”), Polarization Dependent Loss (“PDL”)), thus        enabling to improve the transmission capacity and the reach        distance;    -   3. Provide performance monitoring parameters such as In-Band        Optical Signal to Noise Ratio (“OSNR”), Electrical Signal to        Noise Ratio (“ESNR”), accumulated CD, PMD and PDL of the        detected signal;    -   4. Adaptively reconfigure signal detection strategies in order        to cope with dynamic networks;    -   5. Enable the use of Soft Decision Forward Error Correction        (SD-FEC) techniques to enable increasing impairment resiliency.

With the shift towards advanced coherent modulation formats and the useof DSP, high spectral efficiency optical networks may be designed withalmost no restriction on accumulated CD and PMD. Current technologiesenable compensation of up to +/−60 000 ps/nm accumulated CD and 30 ps ofPMD. Consequently, the transmission reach is limited mainly by theAmplified Spontaneous Emission (“ASE”) noise from the optical amplifiersand the optical nonlinear effects.

Real time monitoring of the OSNR is a requirement set to ensuresatisfactory signal quality and to monitor potential failures at thetransmission link. Several methods have been proposed in the art toderive the In-Band OSNR level by estimating the in band noise leveldirectly, even in the presence of optical filters in the link. Thesemethods are in compliance with the use of polarization multiplexing andcoherent optical modulation formats. Two methods for In-Band OSNRmonitoring based on Stimulated Brillouin Scattering (“SBS”) effect havebeen described in the Applicant's patent applications published under US20120063772 and US 20120219285 and are hereby incorporated by reference.

Other methods which rely upon the use of the DSP in a coherent receiverhave also been proposed. For example, Z. Dong, A. P. T Lau and C. Lu, in“OSNR monitoring for QPSK and 16-QAM systems in presence of fibernonlinearities for digital coherent receivers”, Optics Express, vol. 20,no. 17, pp. 19520-19534, 2012, describe a method forfiber-nonlinearity-insensitive OSNR monitoring in digital coherentreceivers, which uses incorporating and calibrating fibernonlinearity-induced amplitude noise correlations among neighboringsymbols into conventional OSNR estimation techniques from receivedsignal distributions.

However the monitoring of the OSNR level of the signal is still notsufficient in order to monitor the overall OSNR system margin. Theoverall OSNR system margin is defined as the margin in term of OSNR fromthe current operating OSNR level of the channel, to the OSNR level thatis attained for a given pre FEC Bit Error Rate (“BER”) target. Usually,this is the pre FEC BER threshold for which the post FEC BER is 10⁻¹⁵.Link induced physical degradations, such as received optical power tothe receiver, CD, PMD, PDL and more specifically nonlinear effects, canchange significantly the OSNR level to be attained for a given BERtarget and therefore causes difficulties in the estimation of theoverall OSNR system margin.

Monitoring the OSNR system margin is required in different phases of theoptical network operation, starting from the link commissioning (whereone needs to compare actual and expected system margin based on thenetwork design and to proceed therefrom to the necessary adaptions ifrequired), in-traffic operation (in order to monitor potential systemdegradations and to make the necessary link adaptions and/or signalrerouting if required), and in failure detection (in order to localizethe fault location).

OSNR system margin monitoring is particularly required when usingsoftware defined optical coherent transceivers, in order to optimize thetransceiver adaptive parameters such as the bit rate, symbol ratemodulation formats and FEC overhead, as part of the service and networkrequirements such as reach distance, capacity, service priority andlatency.

A conventional prior art method for OSNR system margin monitoring isillustrated in FIG. 1. After being sent along the network link, aportion of the signal to be monitored is tapped out from the link andthe signal OSNR level is measured using an Optical Spectral Analyzer(“OSA”), followed by providing by the receiver's FEC decoder module thepre FEC BER level. In order to reach the pre FEC BER target, the signalOSNR level is deliberately deteriorated prior to reaching the receiver,by using two Erbium Doped Fiber Amplifiers (EDFA) set in a cascadeconfiguration with a Variable Optical Attenuator (VOA) that acts as aspan loss element located between the two amplifiers. Such approach hasthe disadvantage of having to use complex and expensive networkequipment that prevents its systematic use. Therefore, when it isrequired according to this method to measure OSNR margin at a givennetwork node, one might need to bring these equipments to the geographiclocation of the node (incurring significant operational expenses) and tofind a monitor access point at the link where the signal may be tappedout while avoiding traffic disturbances during the measurement.

U.S. Pat. No. 7,561,797 describes a method and system for controllingOSNR of an optical signal at a receiver end of an optical link. Theproposed method is based on degrading the signal at the TX side byimplementing one of the following two options:

Option 1: a pre-compensating function is used for a digital filter (e.g.in order to pre-compensate the CD) while leaving a residual impairmentat the receiver side (e.g. a residual chromatic dispersion).

Option 2: adding at the transmitter side, a digital electrical noise tothe digital electrical signal to be transmitted before the Digital toAnalog Converter (“DAC”) operates thereon. However, U.S. Pat. No.7,561,797 has the disadvantages that it requires to degrade the signalbefore its transmission via the link, and that information is requiredto be sent from the receiver, back to the transmitter, via a controlchannel in order to control the degradation strength. In addition,deriving the in band OSNR based only on the pre FEC BER measurement, isnot accurate enough since the pre FEC BER is proportional to theelectrical Signal to Noise Ratio (ESNR) and the ESNR and OSNR arelinearly proportional only when the OSNR level is low enough and whenthe only predominant impairment in the link comes from the ASE noise. Incase of a nonlinear impairment and/or CD and PMD impairments, the linearrelationship between the ENSR and OSNR is not valid.

Therefore an accurate OSNR system margin monitoring method is required,which should be robust to link impairments such as fiber nonlinearities,CD, PMD and PDL. Such a method should not affect the signal servicequality and should enable remote monitoring operation in order for it tobe cost effective.

SUMMARY OF THE DISCLOSURE

It is an object of the present disclosure to provide a novel, relativelyinexpensive method, insensitive to physical link impairments for OSNRsystem margin monitoring in optical networks.

It is another object of the present disclosure to provide a method whichcomplies with coherent modulation formats which can be carried outtransparently, without affecting the signal quality and withoutrequiring installation of complex equipment at the network nodes.

Other objects of the invention will become apparent as the descriptionof the invention proceeds.

The relationship between the OSNR and ESNR for coherent optical signalsis described according to the publication “On the nonlinear distortionsof highly dispersive optical coherent systems” by F. Vacondio et al.(Optics Express, vol. 20, no. 2, pp. 1022-1032, 2012) as follows:

${ESNR} = \frac{A \times {OSNR}}{1 + {K \times {OSNR}}}$

wherein the parameter “A” is associated with the transmissioncharacteristics (e.g. symbol rate, modulation format, bandwidth of thereceiver's electrical filter, and optical bandwidth of the opticalfilter before detection of the signal) whereas the parameter “K” (beingthe overall physical impairment strength parameter) refers to thesaturation effect that exists between the OSNR and the ESNR (forOSNR>>1/K, the ESNR approaches the value of A/K) and is associated withthe physical impairments of the link along which the coherent opticalsignals have been conveyed such as optical received power, CD, PMD, PDLand nonlinear fiber effects.

According to a first aspect of the disclosure, there is provided amethod for monitoring OSNR system margin in optical networks, whichcomprises the steps of:

(i) receiving a coherent optical signal in a digitalized form;

(ii) obtaining a value of a current OSNR, (OSNR_(dB)), associated withthe received coherent optical signal;

(iii) obtaining a value for a current ESNR, (ESNR_(1,dB)), associatedwith the received coherent optical signal;

(iv) determining a value for a reference ESNR, (ESNR_(2,dB));

(v) retrieving a value for a parameter A that is associated withtransmission characteristics that relate to the received coherentoptical signals;

(vi) calculating a value for a parameter K that is associated withphysical impairments of a channel along which the coherent opticalsignal was received; and

(vii) determining a value of the OSNR system margin, (ΔOSNR_(dB)), basedon current values of ESNR_(1,dB), ESNR_(2,dB), parameter A and parameterK and deriving there from changes that occur in said OSNR system marginbeing monitored.

According to another embodiment, step (vii) further comprisescalculating the ESNR margin, ΔESNR_(dB), by:ΔESNR_(dB)=ESNR_(1,dB)−ESNR_(2,dB)

In accordance with another embodiment, a value of the OSNR systemmargin, (ΔOSNR_(dB)), is determined by applying the following equation:

${\Delta\;{OSNR}_{d\; B}} = {{\Delta\;{ESNR}_{d\; B}} - {10{\log_{10}\left( {A - {K \times 10^{\frac{{ESNR}_{1,{d\; B}}}{10}}}} \right)}} + {10{\log_{10}\left( {A - {K \times 10^{\frac{{ESNR}_{2,{d\; B}}}{10}}}} \right)}}}$

By yet another embodiment, the value of the reference ESNR,(ESNR_(2,dB)), is determined in step (iv) by adding digital noise to thecoherent optical signal being in a digitalized form, until the value forESNR_(2,dB) is such that the value of the pre-defined BER target isreached.

According to still another embodiment, the value of the reference ESNR,(ESNR_(2,dB)), is retrieved from a database (e.g. a look up table).

In accordance with another embodiment, step (vi) is carried out bycalculating the value of parameter K of the channel along which thecoherent optical signal was received by:

$K = {\frac{A}{10^{\frac{{ESNR}_{1,{d\; B}}}{10}}} - \frac{1}{10^{\frac{{OSNR}_{d\; B}}{10}}}}$

By yet another embodiment, the method provided further comprises a stepthat when the value of the OSNR system margin, (ΔOSNR_(dB)), of anoptical channel being monitored is below a pre-defined value, a switchover is initiated, whereby traffic would be diverted from that opticalchannel to another optical channel. According to another aspect of thedisclosure, there is provided a coherent receiver apparatus configuredto be used in an optical communication network, and comprising:

a receiver configured to receive a coherent optical signal in adigitalized form;

an OSNR monitor configured to measure a value of current OSNR,(OSNR_(dB)) associated with the received coherent optical signal;

an analyzer (i.e. a spectrum analyzer or a complex signal analyzer)configured to measure a value of current ESNR, (ESNR_(1,dB)), associatedwith the received coherent optical signal;

a processor (e.g. a Digital Signal Processing (DSP)) configured to:

-   -   determine a value for a reference ESNR, (ESNR_(2,dB));    -   retrieve a value for a parameter “A” that is associated with        transmission characteristics that relate to the received        coherent optical signals;    -   calculate a value for a parameter “K” that is associated with        physical impairments of a channel along which the coherent        optical signal was received; and    -   determine a value of the OSNR system margin, (ΔOSNR_(dB)), based        on current values of ESNR_(1,dB), ESNR_(2,dB), parameter A and        parameter K and deriving therefrom changes that occur in the        OSNR system margin being monitored.

The term “an OSNR monitor device” as used herein throughout thespecification and claims, is typically an in-band device which is basedon the functionality of one or more of the following: an opticalspectral analyzer, an RF spectrum analyzer, a delay tap asynchronoussampler or a nonlinear optical device such as an optical parametricamplifier or a stimulated Brillouin ring laser.

According to an embodiment of this aspect of the disclosure, theprocessor is further configured to calculate an ESNR margin, ΔESNR_(dB),by:ΔESNR_(dB)=ESNR_(1,dB)−ESNR_(2,dB)

By yet another embodiment the processor is further configured todetermine a value of the OSNR system margin, (ΔOSNR_(dB)), by applyingthe following relationship:

${\Delta\;{OSNR}_{d\; B}} = {{\Delta\;{ESNR}_{d\; B}} - {10{\log_{10}\left( {A - {K \times 10^{\frac{{ESNR}_{1,{d\; B}}}{10}}}} \right)}} + {10{\log_{10}\left( {A - {K \times 10^{\frac{{ESNR}_{2,{d\; B}}}{10}}}} \right)}}}$

According to still another embodiment, the coherent receiver apparatusfurther comprising a digital noise generator configured to generate adigital noise (e.g. a digital noise having a normal distribution) andwherein the processor is further configured to determine the value ofthe reference ESNR, (ESNR_(2,dB)), by adding and combining the generateddigital noise in a controlled way with the coherent optical signal beingin a digitalized form, until the value for ESNR_(2,dB) is such that avalue of a pre-defined BER target is reached.

In accordance with another embodiment, the processor is furtherconfigured to calculate the value of parameter K of the channel alongwhich the coherent optical signal was received, by applying thefollowing relationship:

$K = {\frac{A}{10^{\frac{{ESNR}_{1,{d\; B}}}{10}}} - \frac{1}{10^{\frac{{OSNR}_{d\; B}}{10}}}}$

According to another embodiment, the processor is further configured toretrieve the value of the reference ESNR, (ESNR_(2,dB)), from adatabase. Preferably, the database comprises entries of reference ESNR,(ESNR_(2,dB)), that depend upon at least one member of a group thatconsists of modulation format of the coherent optical signal received,symbol rate of the coherent optical signal received, and opticalfiltering mode (e.g. colored (optical filter bandwidth) opticalfiltering mode or colorless optical filtering mode).

By yet another embodiment, the source for the noise may be an analogelectrical source that may be added to the coherent optical receivedsignal, prior to its digitalization (e.g. by using an Analog to DigitalConvertor (“ADC”)).

The method described herein enables monitoring of the OSNR system marginof optical coherent signals as well as the overall physical impairmentstrength parameter in real modern optical networks without requiring anexternal ASE optical source. Furthermore, this approach does not requirehuman intervention on site since it can be carried out remotely. It isrobust to physical impairments (especially to nonlinear fiber effects)and provides excellent accuracy.

It should be understood that the method provided is applicable to allcoherent modulation formats, for example, BPSK (Binary Phase shiftKeying), M-ary PAM (Pulse Amplitude Modulation), QPSK (Quaternary PhaseShift Keying), M-ary QAM (Quadrature Amplitude Modulation), and thelike. In addition, the method is also applicable for cases of dualpolarization versions of the above modulation formats, with both singlecarrier (Orthogonal Frequency Division multiplexing) OFDM approaches.

Nonlinear impairments (also referred to as “nonlinear interferencenoise”) that are present in the link, may lead to addition of acircularly Gaussian distributed noise at the recovered signalconstellation after its processing (e.g. by the DSP). This is the casefor non dispersion managed optical links. In such a case, the ESNRreference for the target pre-FEC BER is independent of physical linkimpairments (in contrast to the OSNR reference for the target pre-FECBER). Therefore the use of a lookup table for the reference ESNR may bepreferred.

However, in case the coherent optical signal is being conveyed along adispersion managed link, the nonlinear interference noise distributiondeviates from circular normality. Thus, using a look up table approachto retrieve the reference ESNR therefrom, might lead to someinaccuracies, especially for low target pre-FEC BER levels (<4×10⁻³). Insuch a case, it would be preferred to implement the embodimentsdescribed by which digital noise is generated and added in a controlledway to the signal, until a reference ESNR is found that satisfies apre-defined value of the target pre-FEC BER.

Due to the nature of coherent detection, the operation may also becarried out by using a colorless approach (i.e. without installing anyoptical filter that precedes the receiver). In such a case, a singlemonitor may be used for scanning the OSNR system margin of the opticalcoherent channels that are present in the optical spectrum, only bytuning the local oscillator optical frequency to the respective opticalfrequencies of the optical coherent channels.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, reference isnow made to the following detailed description taken in conjunction withthe accompanying drawings wherein:

FIG. 1 illustrates a prior art system implementing OSNR system marginmeasurement based on the use of a broadband ASE noise source and an OSA;

FIG. 2 illustrates an example demonstrating how physical linkimpairments affect the OSNR system margin;

FIG. 3 illustrates a schematic implementation of an embodiment of thepresent disclosure by which OSNR system margin monitoring is based uponESNR and OSNR monitoring and the use of a lookup table to extract valuesof parameter A and the reference ESNR;

FIG. 4 illustrates a schematic implementation of another embodiment ofthe present disclosure by which OSNR system margin monitoring is basedupon ESNR and OSNR monitoring and the use of a lookup table to extractthe values of parameter A and the reference ESNR;

FIG. 5 demonstrate exemplary experimental results of transmission of2×120.6 Gb/s DP-QPSK channels together with 2×45.8 Gb/s DP-QPSK channelsover a non dispersion managed 5×100 km link;

FIG. 5A presents experimental results of the dependence of ESNR on theOSNR for different launched optical power levels, for 120.6 Gb/s DP-QPSKchannel with a received optical power of −10 dBm;

FIG. 5B presents experimental results for the dependence of ESNR andOSNR thresholds for BER targets of 1.5×10⁻² and 2×10⁻³ respectively, onthe launched optical power levels for 120.6 Gb/s DP-QPSK channel withreceived optical power of −10 dBm;

FIG. 5C presents exemplary experimental results of a comparison heldbetween prior art and the method provided by the present disclosure ofOSNR system margin monitoring as function of the launched power for120.6 Gb/s DP-QPSK channel;

FIG. 6 illustrate exemplary experimental results of transmission of1×120.6 Gb/s DP-QPSK channels together with 2×45.8 Gb/s DP-QPSK channelsand 4×10.7 OOK channels over a dispersion managed 5×100 km of G.652link;

FIG. 6A presents experimental results of the dependence of ESNR on theOSNR for different launched optical power levels for 120.6 Gb/s DP-QPSKchannel with a received optical power of −10 dBm;

FIG. 6B presents experimental results of the dependence of ESNR and OSNRthresholds for BER targets of 1.5×10⁻² and 2×10⁻³ on the launchedoptical power levels for 120.6 Gb/s DP-QPSK channel with a receivedoptical power of −10 dBm;

FIG. 6C presents exemplary experimental result of a comparison heldbetween prior art and the method provided by the present disclosure ofOSNR system margin monitoring as function of the launched power for120.6 Gb/s DP-QPSK channel;

FIG. 7 illustrates schematically another embodiment provided by thepresent disclosure for an OSNR system margin monitoring; and

FIG. 8 illustrates schematically yet another embodiment provided by thepresent disclosure for an OSNR system margin monitoring.

DETAILED DESCRIPTION

In the disclosure, the term “comprising” is intended to have anopen-ended meaning so that when a first element is stated as comprisinga second element, the first element may also include one or more otherelements that are not necessarily identified or described herein, orrecited in the claims. For the purposes of explanation, numerousspecific details are set forth in order to provide a thoroughunderstanding of the present invention. It should be apparent, however,that the present invention may be practiced without these specificdetails.

FIG. 1 illustrates a prior art set-up of OSNR system margin measurementthat relies on the use of a broadband ASE noise source and OSA. Afterconveying the signal through the network's link, a portion of the signalto be monitored is tapped off from the link. The signal OSNR level ismeasured using an Optical Spectral Analyzer (OSA), and the FEC decodermodule of the receiver provides the pre FEC BER level. In order to reachthe pre FEC BER target, the signal OSNR level is deteriorated before thereceiver by using two Erbium Doped Fiber Amplifiers (EDFAs) in a cascadeconfiguration with a Variable Optical Attenuator (VOA) that acts as aspan loss compensating element located between the two amplifiers. Suchan approach has the disadvantages of requiring the use of complex andexpensive network equipment that inhibit operator from its usethroughout their systems. Therefore, according to the prior artsolution, when one is required to carry out OSNR margin measurements ata given network node, one may need to physically bring these pieces ofequipment to the geographic location of the node (leading to significantoperational expenses) and to find a monitor access point at the linkwhere the signal can be tapped off without introducing trafficinterference while taking these measurements.

FIG. 2 presents two scenarios of the dependency of the pre FEC BERtarget of 1.5×10⁻² on the OSNR in an example where a single channel of120.6 Gb/s DP-QPSK is transmitted over five spans, each one of thesespans comprises 100 km of standard single mode fiber (which is incompliance with ITU-T Recommendation G.652):

-   -   Scenario 1: launched power per span is 1 dBm and the received        optical power is −10 dBm (linear transmission case); and    -   Scenario 2: launched power per span is 7 dBm and the received        optical power is −19 dBm (nonlinear transmission case).

The pre FEC BER target is reached at OSNR=12.6 dB for scenario 1, andOSNR=14.5 dB for scenario 2. Assuming that the OSNR at the link end is20 dB, the OSNR system margin is 7.4 dB and 5.5 dB for scenarios 1 and2, respectively. Such an example demonstrates the importance of havingan accurate estimation of the OSNR system margin, as it is verysensitive to the working conditions of the system.

FIG. 3 illustrates a schematic view of an embodiment of the presentdisclosure of an OSNR system margin monitoring based on ESNR and OSNRmonitoring and the use of a database to extract the required valuesassociated with parameter A and the reference ESNR. After being conveyedalong the network link, the optical signal arrives at its terminationpoint and is forwarded to the coherent receiver. Before the coherentdetection, the optical signal may be optionally filtered using anoptical filter (colored detection) or it can be detected without passingthrough an optical filter (colorless detection). In the latter case, allthe optical signals conveyed via the channels present in the fiber, areforwarded to the optical receiver. Colorless detection is not harmfulsince this is the correct selection of the local oscillator frequencythat determines which channel is coherently detected. After convertingthe optical signal to the electrical domain, it is digitalized usingfour high speed ADCs and sent to a DSP block in order to compensate forfiber impairments such as accumulated CD, polarization crosstalks, PMDand PDL. Digital compensation of the frequency carrier offset andoptical phase noise may also be performed. After applying theseimpairment compensation algorithms, the noisy symbols are recovered andare estimated using hard or soft detection techniques. The ESNR isestimated based on the hard or soft symbol decision and the FEC decoderblock has the ability to provide the pre FEC BER.

The OSNR system margin monitor acquires from the management unitnecessary information that relates to the signal to be monitored, suchas the pre FEC BER target, modulation format, symbol rate and opticalfiltering mode: i.e. colored or colorless (meaning, subjecting or notthe arriving signal to an optical filtering prior to its arrival at thereceiver). In the colored mode case, the information of the opticalfilter bandwidth is also provided. With this information, OSNR systemmargin monitor extracts from a lookup table the value of the A parameterand the ESNR at the target pre FEC BER, referred to throughout thespecification and claims as ESNR_(2dB) or ESNR_(ref,dB). In addition,the OSNR system margin monitor sends a request to an external inbandOSNR monitor module, in order to get the OSNR level of the channel to bemonitored. It also sends a request to the coherent receiver in order toget the ESNR level of the detected channel, denoted ESNR_(1dB).

Using the following equation,ΔESNR_(dB)=ESNR_(1,dB)−ESNR_(ref,dB)the OSNR margin monitor can evaluate the ENSR margin of the channel. Theoverall physical impairment strength parameter (denoted K parameter) isevaluated using the following equation:

$K = {\frac{A}{10^{\frac{{ESNR}_{,{1d\; B}}}{10}}} - \frac{1}{10^{\frac{{OSNR}_{d\; B}}{10}}}}$

This parameter takes into account the combination of different physicalimpairments associated with the link, such as the optical detectedpower, the residual CD, residual PMD, residual PDL, that are notcompensated by the DSP of the coherent receiver, as well as thenonlinear impairment (or residual nonlinear impairment if a nonlinearcompensation equalizer is used). Finally, using ESNR_(1,dB),ESNR_(ref,dB), A and K parameters, the OSNR margin, the value of theΔOSNR_(dB) may be derived by using:

${\Delta\;{OSNR}_{d\; B}} = {{\Delta\;{ESNR}_{d\; B}} - {10{\log_{10}\left( {A - {K \times 10^{\frac{{ESNR}_{1,\;{d\; B}}}{10}}}} \right)}} + {10{\log_{10}\left( {A - {K \times 10^{\frac{{ESNR}_{{ref},{d\; B}}}{10}}}} \right)}}}$

The values of the ΔOSNR_(dB) and/or of parameter K (associated with theoverall physical impairment strength parameter) may be returned to themanagement unit for use in its supervision or for taking a furtheraction, if needed. The OSNR system margin monitor can be a part of theembedded software of a transceiver/combiner card that contains thecoherent transmitter/receiver line module or it may be integrated withina DSP unit comprised in the coherent receiver.

Due to the nature of the coherent detection, the monitoring of the OSNRmargin may also be carried out by applying a, colorless approach (i.e.without using any optical filter before the signal arrives at thereceiver). In such a case, a single proposed monitor may be used forscanning the OSNR system margin of the optical coherent channels presentin the optical spectrum, simply by tuning the local oscillator opticalfrequency to the corresponding optical frequencies of the channels.

FIG. 4 is a schematic illustration of another embodiment of the presentdisclosure of the OSNR system margin monitor based on ESNR and OSNRmonitoring and the use of a lookup table to extract the A and theESNR_(ref) parameters. In the present embodiment, the OSNR monitoring isperformed within a DSP block of the coherent receiver.

FIG. 5 exemplify experimental results of 2×120.6 Gb/s DP-QPSK channelsbeing co-transmitted with 2×45.8 Gb/s DP-QPSK channels over a nondispersion managed link consisting of 5×100 km of fiber that complieswith ITU-T Recommendation G.652. The channels are 50 GHz spaced fromeach other and the OSNR margin is measured using the method described inFIG. 3 for one of the two 120.6 Gb/s channels.

FIG. 5A presents experimental results demonstrating the dependency ofthe ESNR on the OSNR for different launched optical power levels for120.6 Gb/s DP-QPSK channel with a received optical power of −10 dBm. Itmay be seen that the ESNR dependency on the OSNR matches the followingrelationship:

${ESNR} = \frac{A \times {OSNR}}{1 + {K \times {OSNR}}}$

As the launched power per span increases, the channel undergoes highernonlinear impairment which is translated into nonlinear interferencenoise in the coherent detection. The dashed curves illustrate anexcellent fitting with the measurement and the following A, and Kparameters were obtained as functions of the launched power:

Launched power per span P = 1 dBm P = 6 dBm P = 7 dBm A 0.41 0.41 0.41 K0.009 0.018 0.025

It may be seen that in non dispersion managed links, only the Kparameter is affected by the nonlinear impairments, while A parameterremains constant. Parameter A depends only on the link characteristicssuch as the symbol rate, modulation format, filtering mode, andtherefore the A parameter values can be stored at a lookup table havingthe following entries: the symbol rate, modulation format and filteringmode.

FIG. 5B illustrates experimental results of the dependency between therequired ESNR and OSNR thresholds for BER targets of 1.5×10⁻² and2×10⁻³, on the launched optical power levels per span for a 120.6 Gb/sDP-QPSK channel having a received optical power of −10 dBm. It may beseen that for both pre FEC BER targets, the required ESNR is independentof the launched power per span, whereas the OSNR that is required toreach the pre FEC BER target, increases along with the launched powerper span, as a result of the nonlinear impairments. When the opticallaunched power is increased from 1 dB to 7 dB, the required OSNR levelfor 1.5×10⁻² pre FEC BER target increases by 1.3 dB, while the requiredOSNR level for 2×10⁻³ pre FEC BER target increases by 3.05 dB. The ESNRrequired for reaching the respective pre FEC BER target is independentof the launched optical power per span since in non-dispersion managedlink, the nonlinear interference noise is typically circularly symmetriccomplex Gaussian distributed as the ASE noise and therefore cannot bedistinguished from the ASE noise. Therefore, the required ESNR valuesmay be acquired in a back to back set up in a colored or colorlessfiltering mode configuration (i.e. without a fiber but with an ASE noisesource, in order to tune the OSNR as well as the ESNR levels), and bestored at a lookup table having entries that are function of the pre FECBER target, symbol rate, modulation format and filtering mode.

FIG. 5C exemplifies an experimental comparison between the resultsobtaining by following the prior art method illustrated in FIG. 1, andthe results obtained by implementing the proposed method of OSNR systemmargin monitoring, as a function of the launched power for a 120.6 Gb/sDP-QPSK channel, for pre FEC BER targets of 1.5×10⁻² and 2×10⁻³,respectively. It is assumed that the channel OSNR at the receiver isfixed at 20 dBm, the ESNR reference values are set to 8.15 dB and 10.1dB for the pre FEC BER levels of 1.5×10⁻² and 2×10⁻³ respectively, andthe optical received power is −10 dBm. The figure presents a good matchbetween the two methods, with an error below 0.2 dB for both pre FEC BERtargets.

FIG. 6 exemplify experimental results of 1×120.6 Gb/s DP-QPSK channelsbeing co-transmitted with 2×45.8 Gb/s DP-QPSK channels and 4×10.7 OOKchannels over a dispersion managed link consisting of 5×100 km of fiberthat complies with ITU-T Recommendation G.652. The channels are 50 GHzspaced from each other and there is a guard band of 300 GHz between the120.6 Gb/s DP-QPSK channel and the 10.7 Gb/s channels. For each of thetwo first spans, a dispersion compensation fiber (DCF) was used at thespan end to compensate for the 90 km of CD, while for each of the lastthree spans a DCF compensating for the 95 km is used.

FIG. 6A illustrates experimental results of the ESNR dependency on theOSNR, for different launched optical power levels for a 120.6 Gb/sDP-QPSK channel with a received optical power of −10 dBm. As thelaunched power per span increases, the channel undergoes more nonlinearimpairment which is translated into nonlinear interference noise aftercarrying out the coherent detection. The dashed curves in this Fig.present an excellent fitting with the measurement and the following Aand K parameters were obtained as a function of the launched power:

Launched power per span P = 0 dBm P = 3 dBm P = 4 dBm P = 5 dBm A 0.410.41 0.41 0.41 K 0.009 0.0155 0.0195 0.0275

It may be noted that in dispersion managed links, only the K parameteris affected by the nonlinear impairments while A remains constant.Parameter A depends only on the back to back characteristics of the link(e.g. the symbol rate, modulation format, filtering mode) and thereforeits value can be stored at a lookup table having the entries: symbolrate, modulation format and filtering mode.

FIG. 6B demonstrates the experimental results of the dependency of ESNRand OSNR thresholds, for pre FEC BER targets of 1.5×10⁻² and 2×10⁻³, onthe launched optical power levels, for 1a 20.6 Gb/s DP-QPSK channel witha received optical power of −10 dBm. It may be seen that the OSNRrequired for reaching the pre FEC BER target increases along with thelaunched power per span, as a result of the nonlinear impairments. Whenthe optical launched power is increased from 0 dB to 5 dB, the requiredOSNR level for 1.5×10⁻² pre FEC BER target increases by 2.25 dB whilethe required OSNR level for 2×10⁻³ pre FEC BER target increases by 5.6dB. It may be seen that for a case of dispersion managed links, the ESNRrequired for reaching the pre FEC BER target value increases slightlyalong with the launched power per span. When increasing the opticallaunched power from 0 dB to 5 dB, the required ESNR level for 1.5×10⁻²pre FEC BER target increases by 0.25 dB, while the required OSNR levelfor 2×10⁻³ pre FEC BER target increases by 0.35 dB. Unlike the nondispersion managed links cases, the ESNR required for obtaining the preFEC BER target is slightly dependent on the launched optical power perspan, since the nonlinear interference noise distribution deviates fromthe circularly symmetric complex Gaussian distribution. Therefore, onewould expect to get an OSNR system margin error if the required ESNRvalues is acquired from a back to back set up (i.e. without using afiber but with an ASE noise source in order to tune the OSNR as well asthe ESNR levels) and the required ESNR values are stored at a lookuptable which entries are function of the pre FEC BER target, symbol rate,and modulation format. However these errors are expected to besignificant only in a case where the pre FEC DER target is substantiallyless than 4×10⁻³ and for high nonlinear OSNR penalty (e.g. >2.5 dB).

FIG. 6C exemplifies an experimental comparison between the resultsobtaining by following the prior art method illustrated in FIG. 1, andthe results obtained by implementing the proposed method of OSNR systemmargin monitoring, as a function of the launched power for a 120.6 Gb/sDP-QPSK channel. It is assumed that the channel OSNR at the receiver isfixed at 20 dBm and the optical received power is −10 dBm. It is alsoassumed that the ESNR reference values are set to 8.15 dB and 10.1 dBfor pre FEC BER levels of 1.5×10⁻² and 2×10⁻³ respectively, and that theoptical received power is −10 dBm. For a pre FEC BER target of 2×10⁻³,the error is below 0.35 dB for launched power of up to 4 dBm, whereas inthe case of launched power of 5 dBm per span, the error increases to1.35 dB due to the high nonlinear penalty (5.6 dB) and deviation of thenonlinear interference noise distribution from the circularly symmetriccomplex Gaussian distribution.

FIG. 7 illustrates a schematic presentation of another embodiment of thepresent disclosure where the OSNR system margin monitoring is based uponmonitoring the ESNR and the OSNR, and upon the use of a digital noisesource generated at a DSP block of the coherent receiver. A lookup tableis used to retrieve the value of parameter A therefrom. Implementingthis embodiment enables the operator to reduce measurement errors whenthe nonlinear interference noise deviates from circularly symmetriccomplex Gaussian distribution. After being conveyed along the networklink, the optical signal arrives at its termination point and isforwarded to the coherent receiver. As was previously explained, beforethe coherent detection is taking place, the signal may optionally befiltered by using an optical filter (colored detection) or it may bedetected without passing the signal via an optical filter (colorlessdetection). After converting the optical signal to the electricaldomain, the converted signal is digitalized (e.g. by using four highspeed ADCs) and forwarded to a DSP block in order to compensate for thefiber impairments such as accumulated CD, polarization crosstalks, PMDand PDL. Digital compensation of the frequency carrier offset andoptical phase noise may also be performed. A digital noise sourcegenerates independent circular symmetric complex Gaussian noise samplesthat may be added to both polarization tributaries of the sampleddetected signal. After applying the impairment compensation algorithms,the noisy symbols are recovered and are estimated using hard or softdetection techniques. The ESNR is estimated based on the hard or softsymbol decision and the FEC decoder block has the ability to provide thepre FEC BER. The amplitude of the digital noise samples is set so thatthe detected recovered symbols reach a given ESNR level.

The OSNR system margin monitor acquires from the management unit thenecessary information that relates to the signal being monitored such asits modulation format, its symbol rate and its optical filtering mode.In case of a colored optical filtering mode, the information of theoptical filter bandwidth may also be provided. The information thusobtained may then be used by the OSNR system margin monitor to retrievefrom a lookup table the value of the A parameter. In addition, anexternal in-band OSNR module provides the OSNR level of the channelbeing monitored. The OSNR margin monitor sends a request to the coherentreceiver in order to be provided with the ESNR level of the detectedsignal (channel), and the value ESNR_(1dB) is provided when the digitalnoise source is disconnected so as not to affect the detected signalsamples. Then, the OSNR margin monitor sends a second request to thecoherent receiver in order to deteriorate the ESNR level for reachingthe pre FEC BER target value (information that is provided by themanagement unit to the optical receiver) by adding the digital samplednoise generated by the digital noise source. The value of the ESNR levelobtained for the pre FEC BER target is substituted in the relevantequation provided hereinbefore as ESNR_(2dB). The digital noise sourceadapts the amplitude of the added noise samples according to theobtained pre FEC BER monitor, in order to reach the pre FEC BER targetvalue. The ESNR level of the detected channel, ESNR_(1dB), is obtainedas explained above when the digital noise source is disconnected fromthe detected signal samples. The OSNR margin monitor may evaluate theENSR margin of the channel and the overall physical impairment strengthparameter (the K parameter) is evaluated. Finally, using ESNR_(1,dB),ESNR_(2,dB), A and K parameters, the OSNR margin, ΔOSNR_(dB), iscalculated and may be sent back to the management unit for supervisionor for further action if needed. The OSNR system margin monitor may be apart of an embedded software of a transceiver/combiner card thatcontains the coherent transmitter/receiver line module, or may beintegrated within a DSP unit of the coherent receiver.

Due to the nature of the coherent detection, the operation may also becarried out by applying a colorless approach, and in such a case, asingle proposed monitor may be used for scanning the OSNR system marginof the optical coherent channels present in the optical spectrum, bytuning the local oscillator optical frequency to the respective opticalfrequencies of the arriving channels.

FIG. 8 illustrates a schematic presentation of another embodiment of thepresent disclosure where the OSNR system margin monitoring is based uponmonitoring the ESNR and the OSNR, upon the use of a digital noise sourcegenerator located at the DSP block of the coherent receiver and a lookuptable to extract the value of parameter A therefrom. In this embodiment,the OSNR monitoring is carried out within a DSP block comprised in thecoherent receiver.

In the description and claims of the present application, each of theverbs, “comprise” “include” and “have”, and conjugates thereof, are usedto indicate that the object or objects of the verb are not necessarily acomplete listing of members, components, elements or parts of thesubject or subjects of the verb.

The present invention has been described using detailed descriptions ofembodiments thereof that are provided by way of example and are notintended to limit the scope of the invention in any way. The describedembodiments comprise different features, not all of which are requiredin all embodiments of the invention. Some embodiments of the presentinvention utilize only some of the features or possible combinations ofthe features. Variations of embodiments of the present invention thatare described and embodiments of the present invention comprisingdifferent combinations of features noted in the described embodimentswill occur to persons of the art. The scope of the invention is limitedonly by the following claims.

What is claimed is:
 1. A method for determining an opticalsignal-to-noise ratio (OSNR) system margin in an optical network,comprising: receiving a coherent optical signal; determining an OSNR andan electrical signal-to-noise ratio (ESNR) associated with the receivedcoherent optical signal; determining a reference ESNR for the receivedcoherent optical signal; determining a first parameter based on atransmission characteristic of the received coherent optical signal anda second parameter based on a physical impairment for an optical channelfrom which the coherent optical signal was received; determining theOSNR system margin based on the first parameter and second parameter;and monitoring changes over time in the OSNR system margin.
 2. Themethod of claim 1, further comprising determining an ESNR margin bycomparing the determined ESNR with the reference ESNR.
 3. The method ofclaim 2, wherein the OSNR system margin is determined based on the ESNRmargin.
 4. The method of claim 1, wherein the first parameter isretrieved from a database.
 5. The method of claim 4, wherein thedatabase comprises an entry of the first parameter depending upon atleast one of: a modulation format of the received coherent opticalsignal, a symbol rate of the received coherent optical signal, and anoptical filtering mode.
 6. The method of claim 1, wherein the referenceESNR is retrieved from a database.
 7. The method of claim 6, wherein thedatabase comprises an entry of the reference ESNR depending upon atleast one of: a pre-defined bit error rate (BER) target, a symbol rateof the received coherent optical signal, a modulation format of thereceived coherent optical signal, and an optical filtering mode.
 8. Themethod of claim 1, wherein the reference ESNR is determined byincreasing noise in the received coherent optical signal until apre-defined BER is reached.
 9. The method of claim 1, wherein the secondparameter is determined based on the first parameter.
 10. The method ofclaim 1, further comprising switching to another optical channel whenthe OSNR system margin is below a pre-defined threshold.
 11. Anapparatus for determining an OSNR system margin in an optical network,comprising: a receiver configured to receive a coherent optical signalin a digitalized form; at least one monitor configured to determine anOSNR and an ESNR associated with the received coherent optical signal;and at least one processor configured to: determine a reference ESNR;determine a first parameter based on a transmission characteristic ofthe received coherent optical signal and a second parameter based on aphysical impairment of a channel from which the coherent optical signalwas received; and determine the OSNR system margin based on the firstparameter and second parameter; and monitor changes over time in theOSNR system margin.
 12. The apparatus of claim 11, wherein the at leastone processor is further configured to determine an ESNR margin bycomparing the determined ESNR with the reference ESNR.
 13. The apparatusof claim 12, wherein the at least one processor is further configured todetermine the OSNR system margin based on the ESNR margin.
 14. Theapparatus of claim 11, wherein the first parameter is retrieved from adatabase.
 15. The apparatus of claim 14, wherein the database comprisesan entry of the first parameter depending upon at least one of: amodulation format of the received coherent optical signal, a symbol rateof the received coherent optical signal, and an optical filtering mode.16. The apparatus of claim 11, wherein the reference ESNR is retrievedfrom a database.
 17. The apparatus of claim 16, wherein the databasecomprises an entry of the reference ESNR depending upon at least one of:a pre-defined BER target, a symbol rate of the received coherent opticalsignal, a modulation format of the received coherent optical signal, andan optical filtering mode.
 18. The apparatus of claim 11, furthercomprising a digital noise generator configured to generate a digitalnoise and wherein the at least one processor is further configured todetermine the reference ESNR by combining the generated digital noisewith the received coherent optical signal until a pre-defined BER targetis reached.
 19. The apparatus of claim 11, wherein the at least oneprocessor is further configured to determine the second parameter basedon the first parameter.
 20. The apparatus of claim 11, wherein the atleast one processor is further configured to switch to another opticalchannel when the OSNR system margin is below a pre-defined threshold.